1. Technical Field
The present invention relates to radio telecommunications, and more particularly to an apparatus and method for improving the efficiency of amplifiers used in mobile radio telephones.
2. Background Art
Wireless cellular telecommunications systems typically utilize radio frequency signals to exchange information between fixed base stations and free-standing mobile radio telephones. Each base station transmits and receives modulated signals to and from the telephones within a given geographic area through a base station antenna. The base station antenna operates over a fixed-area range called a xe2x80x9ccell.xe2x80x9d In order to provide service to mobile radio telephones over a larger geographic area, several cells are positioned over the area to provide complete coverage.
In typical cellular configurations, base station antennae produce signals that propagate outward 360 degrees from the antenna. When a mobile radio telephone is transmitting radio frequency signals to a base station antenna, the strength of the transmitted signal must be strong enough to be received by the base station antenna even when the mobile radio telephone is located at the most distant fringes of the cell.
Most mobile radio telephones utilize a driver stage which incorporates driver amplifier components. In order to ensure the strength of the transmission signal at distant points within larger cells, some mobile radio telephones utilize a separate power amplifier in the output stage of the telephone circuitry to boost power to the modulated signal delivered from the driver stage. When the mobile radio telephones are located close to the base station antenna near the center of a cell, however, a stronger transmission signal is not needed. Boosting transmission signals in such situations where high power transmission is not necessary wastes valuable battery power within a mobile radio telephone. The net result of such inefficient power consumption and poor transmitter efficiency is a reduced amount of talk time for the user.
In the past, the radio frequency drive level to the driver stage was lowered when less than full power was required from the power amplifier. This would cause the efficiency of the power amplifier to decrease significantly. Furthermore, because the output transistors of the power amplifier are sized for optimum efficiency at full power output, considerable current could be wasted in the power amplifier when during operation at lower power levels. In some prior art mobile radio telephones, bias to the power amplifier would be switched to a lower idling current when a low output power was required. At such low output levels, the driver amplifier within the driver stage would itself be capable of exciting the telephone antenna to effectively transmit radio signals. Thus, if the power requested from the power amplifier was low enough, the driver stage itself would consume enough DC power to supply the required transmission power directly to the antenna, and the power amplifier would be a redundant drain on valuable current.
Compounding the above problems relating to transmission power, transmitter efficiency can also suffer when the mobile power amplifier is switched between a linear mode and a saturated mode. Conventional analog cellular telephones transmit voice information via a frequency modulation technique, in which an information signal is encoded into a carrier radio signal. In order to efficiently amplify a frequency modulated signal for transmission to a base station, amplifiers within the telephone circuitry are typically operated in a saturation mode to maximize transmission power. Digital cellular telephones and networks, however, transmit digitally modulated information signals in a linear mode. These signals are modulated using a composite modulation technique in which encoded information is carried by both an amplitude modulation and phase variation of the signal. In order to preserve the waveform shape and the integrity of the modulated information during transmission, amplifiers must be operated in a linear mode.
Digital cellular networks and subscriber services are not as widely available as analog networks in many geographical areas. A user, therefore, will often be forced to use a digital-ready mobile telephone within an analog-only network. In order for a digital mobile radio telephone to be usable within both digital and analog cellular networks, prior art mobile telephones have incorporated switchable amplifier stages to allow the mode of transmission to change between a saturated and a linear mode to accommodate either analog and digital transmissions, respectively.
An example of such a system is shown in U.S. Pat. No. 5,060,294. In this prior art system, the information signal modulation is detected and signal amplifier circuitry is accordingly operated in either a saturation mode or linear mode. This system has drawbacks, however, because the driver stage, the output stage and an impedance matching network implemented in the circuitry are switched together between the saturation mode or linear mode. Because the driver stage must be shifted into each operating mode along with the output stage, the driver stage is not utilized in an efficient manner, and power consumption within the mobile radio telephone can again suffer.
Therefore, there is a need for an improved method and apparatus for modifying the mode of transmission and for varying the signal power output of mobile radio telephones to use system power more efficiently.
To address these problems and shortcomings in the prior art, an apparatus is provided herein for supplying a radio frequency output to promote higher amplifier efficiencies. The apparatus includes driver means having a driver output for providing an amplified radio frequency signal and switching means in communication with the driver output of the driver means. The switching means directs the amplified radio frequency signal to one of at least a first and a second circuit paths connectable to the output of the driver means. The first circuit path provides a low power level output of the radio frequency signal, and the second circuit path provides a high power level output of the radio frequency signal. In the low power level mode, the power amplifier in the output stage is switched out of the circuit path.
The invention may be further embodied in an apparatus for delivering a radio frequency signal. The apparatus comprises a driver stage having an input receiving a radio frequency signal and an output providing a first amplified radio frequency signal. A switching device is included and is in communication with the driver stage. An output stage is provided and is in communication with the switching device. At least one bypass circuit path in communication with the switching device is included to bypass the output stage and provide the first amplified radio frequency signal directly to the antenna. The switching device is operable to direct the first amplified radio frequency signal from the driver stage to the output stage or the bypass circuit path.
The invention may also be embodied in a method for efficiently amplifying a modulated radio frequency signal in a radio telephone. The method includes the steps of supplying a modulated signal from a driver stage having at least one radio frequency amplifier, determining the transmission power requirements of the modulated signal, selecting one of a plurality of output stages in accordance with the transmission power requirements, switching the modulated signal to the selected output stage, and supplying the output of the selected output stage to an antenna.
The invention may be further embodied in a method for amplifying a modulated radio frequency signal modulated using at least one of a plurality of modulation techniques. The method includes the steps of supplying a modulated signal from a driver stage, determining the modulation technique of the modulated signal, selecting one of a plurality of output stages based on the modulation technique, and supplying the output of the selected output stage to an antenna. The first of the output stages includes at least one power amplifier for use with a composite modulated signal, and at least a second of the output stages does not include a power amplifier. The second output stage is used with frequency modulated signals.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. The invention, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.